Butterflies generally hold their wings erect when they are at rest, with their two upper surfaces in proximity, the under surfaces alone displaying their colors to the eye. Only in a few genera of the larger butterflies, and these tropical species, with which this book does not deal, is there an exception to this rule, save in the case of the _Hesperiidæ_, or "skippers," in which very frequently, while the anterior wings are folded together, the posterior wings lie in a horizontal position.
_Internal Organs._--Thus far we have considered only the external organs of the butterfly. The internal organs have been made the subject of close study and research by many writers, and a volume might be prepared upon this subject. It will, however, suffice for us to call the attention of the student to the principal facts.
[Illustration FIG. 41.--Longitudinal section through the larva of _Anosia plexippus_, ♂, to show the internal anatomy (the Roman numerals indicate the thoracic, the Arabic the abdominal segments): _b_, brain; _sog_, suboesophageal ganglion; _nc_, nervous cord; _oe_, oesophagus; _st_, stomach; _i_, intestine; _c_, colon; _sv_, spinning-vessel of one side; _s_, spinneret; _mv_, Malpighian vessel, of which only the portions lying on the stomach are shown, and not the multitudinous convolutions on the intestine; _t_, testis; _dv_, dorsal vessel; the salivary glands are not shown. (Magnified 3 diameters.) (Burgess.)]
The muscular system finds its principal development in the thorax, which bears the organs of locomotion. The digestive system consists of the proboscis, which has already been described, the gullet, or oesophagus, and the stomach, over which is a large, bladder-like vessel called the food-reservoir, a sort of crop preceding the true stomach, which is a cylindrical tube; the intestine is a slender tube, varying in shape in different genera, divided into the small intestine, the colon, and the rectum. Butterflies breathe through spiracles, little oval openings on the sides of the segments of the body, branching from which inwardly are the tracheæ, or bronchial tubes. The heart, which is located in the same relative position as the spine in vertebrate animals, is a tubular structure. The nervous system lies on the lower or ventral side of the body, its position being exactly the reverse of that which is found in the higher animals. It consists of nervous cords and ganglia, or nerve-knots, in the different segments. Those in the head are more largely developed than elsewhere, forming a rudimentary brain, the larger portion of which consists of two enormous optic nerves. The student who is desirous of informing himself more thoroughly and accurately as to the internal anatomy of these insects may consult with profit some of the treatises which are mentioned in the list of works dealing with the subject which is given elsewhere in this book.